The present invention relates generally to semiconductor fabrication and, more particularly, to a method and apparatus for reducing consumption of fluid delivered to a backside of a substrate during a cleaning operation.
Cleaning chemistries for single wafer cleaning operations are formulated for specific applications and are designed so that a small amount of the chemistry applied to the surface of the wafer is sufficient for cleaning the surface of the wafer. That is, a thin film of the fluid supplied to the surface of the wafer produces the desired cleaning effects. Because of the high costs for the purchase and the disposal of the cleaning chemistries, it is desired to only use the amount of chemistry that is necessary for effective cleaning.
Applying a thin film of the cleaning chemistry to a top surface of a semiconductor substrate is easily accomplished as gravity is working to assist the process. A small amount of fluid can be puddled on a top surface of the substrate and the substrate can be rotated around its axis to spread the fluid over the surface of the substrate without spinning the fluid off of the wafer surface. The speed of the rotation can be used to control the thickness of the fluid layer. However, the cleaning chemistry can not be applied to the backside of the wafer in this manner as the fluid will be lost. FIG. 1 is a schematic diagram of a wafer having cleaning chemistries applied to a top and a backside of the wafer. Wafer 100 has a thin film applied to a top surface of the wafer by top nozzle 104a. However, bottom surface 106 can not retain the fluid from bottom nozzle 104b, where as much as 95% of the fluid delivered to bottom surface 106 can be lost. Thus, conventional spray-on techniques are not effective for low-volume chemistry cleaning of the wafer backside.
One attempt to minimize the fluid loss associated with cleaning the backside of the wafer is to clean the top side of the wafer and then flip the wafer over to clean the other side. However, the throughput for the cleaning process is cut in half since the cleaning is performed sequentially. Accordingly, this alternative is not a viable one. Another attempt to address the shortcomings of the prior art is to provide a reservoir containing the cleaning chemistry and place the backside of the wafer in contact with a meniscus formed by the cleaning chemistry. FIG. 2 is schematic diagram of a wafer coming into contact with a meniscus of a cleaning solution in a reservoir. Bottom surface 106 of wafer 100 is brought into contact with meniscus 108. Meniscus 108 is formed when the cleaning chemistry is filled to the top of reservoir 110. However, a shortcoming with the use of a reservoir is due to each of the cleaning solutions having different surface tensions. Thus, the meniscus height can be different for each of the cleaning chemistries. Consequently, the distance for the wafer to be brought into contact with the cleaning chemistry will change with the different cleaning chemistries. This configuration is also difficult to implement mechanically. Additionally, the contents of reservoir 110 will have to be changed over time as the cleaning chemistry becomes dirty, which negatively impacts throughput and control of contaminants.
In view of the foregoing, there is a need for a method and apparatus for reducing the volume of cleaning chemistry applied to the backside of a wafer in a single-wafer cleaning tool in a manner that does not negatively impact the throughput or defect rate.